The present invention relates to a rotary body such as an optical disc, in particular relates to an optical disc having a picture pattern printed on one surface thereof.
An optical disc such as a CD (compact disc) usually has a label (a picture) printed on one of its surfaces to indicate the content of the information recorded on the disc. Such a label is visible and recognizable by naked-eyes when the CD is in a stopped condition, but is not visible and recognizable by naked-eyes when the CD is being played in a CD player unless using a special illumination device capable of performing a spot irradiation in synchronism with the rotation of the player.
In order to solve the above problem, U.S. Pat. No. 5,155,605 has disclosed an improvement for easily recognizing a label (picture) on a disc even when the disc is in a rotating condition. According to the U.S. patent, the surface of a disc is used as a holographic plate. Such holographic plate is exposed to an interference fringe light obtained by mutual interference between a light beam irradiating a picture (to be recorded on one surface of a disc) and a coherent reference light beam having a predetermined incidence angle, thereby forming a hologram having the above picture data recorded thereon. Then, simply by rotating the disc (which serves as a hologram having a picture recorded thereon), it is possible for a user to recognize the picture on the surface of the disc, without using any special optical means which is usually otherwise required to be positioned between the user and the disc.
In principle, a reflected light can lead to image-formation only when an irradiation light beam irradiating an object (such as the above picture) and a reference light beam are incident with the same incidence angle. If an irradiation light beam is fixed, then, an incidence angle of the irradiation light beam with respect to the hologram will change with the rotation of the disc. Thus, during each rotation of the disc, a reflected light can lead to image-formation only when the irradiation light beam is incident on to the hologram with a predetermined incidence angle (when the irradiation light beam and a reference light beam are incident on to the hologram with an exactly the same incidence angle). Namely, since during each rotation the image-formation can be effected once when the irradiation light beam is incident on to the hologram with a predetermined incidence angle, a completed image may be obtained in synchronizm with the rotation of the disc, allowing a user to recognize the picture on the disc surface by virtue of residual image effect.
In U.S. Pat. No. 5,155,605, the sun light is used as an irradiation light. Since the sun light is a widely diffused light containing various light components of different frequencies, it also contains a light component which can serve as the above reference light beam. Such reference light beam component is incident on to an object with a predetermined incidence angle.
However, with the method disclosed in U.S. Pat. No. 5,155,605, even when a disc is in a stopped condition, it is still difficult for a user to recognize an object (a picture on the disc) if an incidence angle of an irradiating light beam is not adjusted to a predetermined angle. In addition, if a new picture is to be formed on a disc, it will be necessary to form a new hologram and to adjust a position for the new picture to form on the disc. This, however, has been proved to be very troublesome.